Delectric heating



J. mi. ROBERTSON DIELEGTRIC HEATING Filed Nov. 5. 19

Patented Oct. 25, 1949 UNITED STATES PATENT OFFIQEl DIELECTRIG HEATING John W. Robertson, Englewood, N. J., assigner, by direct.A and mesne assignments, of one-half to Ellis-Foster Company, Montclair, N. J., a oorporation of New Jersey, and one-half to Montclair Research' Corporation,

New Jersey a corporation of 'Application November 5, 1945, Serial No.- 626,881

17 Chlml. (0l. 99-217) This invention relates to dielectric heating and apparatus therefor, and products resulting therefrom, and is more particularly concerned with the heating of non-homogeneous mixtures in a dielectric ileld.

The prior art methods on cooking foods and heating foods has always been approached in a conventional procedure by placing the meat, or egg, r potato, or other food, between two condenser plates. Of course, the heating has always been rather non-uniform but can be made more uniform in some cases by carefully preparing the food to uniform thickness and fairly uniform physical properties in order to give a homogeneous dielectric. Or, the electrodes were carefully shaped so as to give a uniform voltage gradient across every part of the fooda rather precise linathematical and mechanical undertaking.

Such methods do not lend themselves to the heating or cooking of non-homogeneous mixtures. Further, foods are normally very irregular in size and shape and processing methods to be practical must be able to make allowances for these variations very quickly and easily with every batch oi material to be treated.

Among the objects of the present invention lis the utilization of heating methods which may be applied practically to non-homogeneous mixtures, such as 'foods and other materials for causing chemical reaction, for heating, for sterilization, and for other purposes.

objects include such heating which may be practically applied to materials and foods which are normally irregular in size and shape.

Further objects include processed foods and `materials produced by such methods.

Figure 3, another form of apparatus that may be employed.

In accordance with the present invention, the heating is carried out by the application oi' radio frequency power to non-homogeneous mixtures to control the heating etl'eot obtained. It is thus possible to produce substantially uniform heating in non-homogeneous mixtures or to direct enhanced heating effects to a portion of such nonhomogeneous mixtures when this is desired. These methods may be employed in connection with the heat treatment of any materials which may be immersed in or suspended in a liquid medium, and the dielectric properties of such medium are controlled to give the heating effect desired. Such operations may be applied in chemical or other reactions where heat treatment 'is supplied to non-homogeneous materials, such as non-homogeneous mixtures of liquids and solids. By the use of a balanced medium, as for example, by adding an -electrolyte to the medium, it is possible to give such liquid medium the power absorption desired for the particular heating operation in hand. For example, a medium may be so balanced as to have substantially the same Still further objects include apparatus for utilization in such methods.

Other objects and advantages will appear from the more detailed description set forth below, it being understood that such detailed description is given by way of illustration and explanation only, and not by way of limitation, since various changes therein may be made by those skilled in the art without departing from the scope and spirit of the present invention.

In conjunction with that more detailed description there is shown in the accompanying drawing, in

absorption of various concentrations of salt in distilled water; in

Figure 2, a. form of apparatus that may be utilized in carrying out the invention; in,

Figure 1, a graph illustrating the rate of power 5 power absorption as the solids immersed or suspended therein and in this way uniform heating may be carried out. Or if desired, the dielectric properties of the medium may be controlled, as for example, by the addition of a substance such as an electrolyte which modifies its dielectric properties so that the liquid medium may be subjected to a greater heating effect than the immersed or suspended solids if desired, or reversing the procedure so that the immersed or suspended solids may be subjected to a heating eiect greater than that oi' the liquid medium. Or combinations of such methods may be employed. In addition, as will be pointed out below, it is possible to employ methods and apparatus carrying out this invention which enables the 'heating effects to be accentuated at particular 0 cation to the heat treatment of non-homogeneous mixtures of liquids and solids and will be illustrated in this connection with the treatment or foods that represents one of the most desirable 'applications of the methods of the present inventiom Thus foods may be cooked or may be u merely sterilized with minimum heating in the shortest practical time.

In carrying out the invention, the food materials which may for example, be small whole fruits or vegetables or pieces of meat, etc., or chopped up foods, may be heat treated in a liquid medium whose dielectric properties are adjusted to fit the particular heating operation being carried out. The operations may be applied to fruit and vegetable juices, pieces of solid fruits and vegetables, small fruits and vegetables in whole form, etc. It has been found that such products have very rapid dielectric heating properties even though the concentration of salts and sugars is rather high and would be expected to show characteristics of conductors rather than poor dielectrics.

Most food materials are capable of being suitably cooked when surrounded by water or other liquid medium, salt solutions, etc. Numerous salts, and/or electrolytes and other materials may be added to the liquid medium to vary the dielectric properties of the mixture undergoing heat treatment in order to get the desired heating eiect upon the application of radio frequency power. To illustrate this phase of the invention, it may be explained by the utilization of sodium chloride or common salt and its effect on the or to control the properties of the liquid medium so that power absorption decreases with increased temperature or increases with increased temperature, as is found most desirable for the particular operation being carried out. In carrying out these methods, the non-homogeneous materials will desirably be treated in a container of dielectric properties such as glass containers and this enables the operation to be particularly useful in sterilizing food materials without heating them to a point where they are completely processed or cooked or the cooking operations may be completed in such containers.

An example which illustrates the control of the heat treatment that may be carried out is the following. Whole grapes were suspended in salil solutions of varying concentration, and it was found that in distilled water and very dilute concentrations of salt, the grapes heated very rapidly dielectric properties of the medium such as water which is sulcient to illustrate the invention, and particularly desirable in view of the common use of salt in foods. The graph shown in Figure 1 illustrates the rate of power absorption of various concentrations of salt in distilled water in a dielectric field at fairly high voltage and a frequency at approximately 20 mc. It will be noted that distilled water is a very good dielectric and shows a relatively low power absorption under the above stated conditions, and this rate of absorption increases as the salt concentration is increased up to about .03%, after which the power absorption drops off. It is interesting to note that it will be possible to determine on which side of the heating peak the concentration lies because on the lower concentrations the rate of power absorption increased steadily as the temperature was brought up and with higher concentrations beyond the peak, the power absorption steadily dropped 01T during heating. These observations are interesting and important in carrying out these methods but must be applied to particular situations to determine how these peaks of power absorption vary with frequency, with temperature, and with the type of molecule which is present in the solution.

It is important to note that very small concentrations of salt, such as .O3-.04% can be utilized to give a high rate of power absorption and this also makes it possible to use ordinary tap water as a liquid medium since the salt concentrations present in tap water may come within the most desired range.

It is thus possible to control the dielectric properties of the non-homogeneous mixture as for example, solids suspended in a liquid medium, to determine the heating `elect obtained. As illustrated in connection with foods, an aqueous medium containing an added material particularly an electrolyte like salt, may be employed, the latter being added in an amount to give the medium the desired dielectric properties for the particular operation being carried out. The added materials such as electrolyte like salt, may be present in an amount to give substantially uniform heating under the conditions, to give substantially the maximum heating eiect if desired.

and would develop steam and burst open before the surrounding water was heated to an appreciable extent. By raising the salt concentration in the medium, a point was found at which reasonably uniform heating took place throughout the contents of the container. The containers used in such work were wide mouth bottles made from "Duraglas or similar baby food jars. It was also noted that in this low salt-concentration range, boiling was extremely vigorous when the boiling point was reached, apparently due to the fact that the power absorption curve was still on the upswing as the liquid reached boiling. However, when the salt concentration had passed the point at which maximum heating took place and where the power absorption was falling oil? with increased temperature, the boiling was not so vigorous when that point was reached. These considerations enable a control of the heating process to be carried out for any particular opera.- tion, as for example, to prevent boil-overs in cornmercial processing. Heating of both the fruit and the surrounding liquid was still relatively fast up to salt concentrations of the order of 1%.

This example illustrates the utilization of methods by which control of the heating rate and the nal temperatures in any container, such as glass bottles, in which food is being heated for purposes of sterilization or canning, may be controlled.

These methods may be applied to the sterilization of fruits and vegetables, fruit and vegetable juices, etc. Thus peas, beans, carrots, beets, and other foods including both vegetable and animal foods may be processed by heating operations which can be readily controlled to be substantially uniform and complete throughout the containers.

In the processing of such vegetable or other materials as in canning operations, it may be desirable to use a steaming oil" process in which a large volume of steam is developed at the surface of the food or other materials just before sealing. This may be done by bringing a high voltage electrode near the surface and generating large volumes of steam only at the surface after which the sealing operation may be carried out. Such steaming off operation may be readily carried out, as for example, by the utilization of apparatus set forth and described below.

Another feature particularly applicable to the canning of food products is concerned with carrying out the processing so that the finished container is full of liquid. It has been found that apparently due to the rapid internal heating by 'I5 the radio frequency processing. most of the air .or salt solution or other liquid medium which had previously illled the jar was drawn back into the centerof the pieces of food to replace the air and often the jars immediately following such processing operation may be not morethan half full of the liquid medium after cooling. To overcome this eifect, it is desirable to carry out the heating operation ilrst in very dilute salt solutions or distilled water under which conditions, the air is expelled more rapidly than normal when the food pieces are heating more rapidly than the surrounding medium. In such operation, the most rapid rate of heating that can be obtained is desired. As soon as most of the air is expelled, vand the mixture is boiling rapidly, the power may be shut off and the product cooled for a few seconds to lower the level of the liquid. At the proper time, the liquidl level is adjusted either with dilute salt solution or with stronger salt solution, depending upon the final salt concentration wanted in the liquid medium as a whole. The exact point at which such extra liquid medium is to be added can be readily determined by simple experiment, so that ilnally the' liquid medium covers the food after thorough cooling. Additional liquid may` be added at such point to fill the bottle level and then sealing carried out immediately thereafter. Further cooling insures a vacuum above the liquid, as its level drops to the ilnal point.

r if desired, an alternative procedure is to allow the level to drop after the initial processing as pointed out above, followed by liquid addition as suggested above, and then the steaming olf procedure is applied as described above, after 'which sealing is carried out.

After any of these procedures of filling and sealing, a second processing may be given. On commercial scale operation, this probably can best be done twenty-four hours after the first processing; but the exact time will vary with particular operations. In such second processing, the heating rate may be somewhat slower due to the increase in strength of the salt solution, through added salt, or through extraction from the food itself, and the processing may be carried out until the closure of the container shows that pressure inside has been equalized with that on the outside by noting the curvature of the lid. This second processing operation can also be carried out under pressure in a large pressure chamber in order to heat the contents of the jar to a higher temperature before equalizing the pressures in the jar and the surrounding air. An

yoperation of this character enables the canner lto choose a temperature for final processing which is suillcient to sterilize completely the particular food being processed.

Another variation of processing which may be utilized gives improvement in sterilization over that involved in heating from room temperature to boiling at atmospheric pressure. This variaall` bacterial activity, without carrying the heating to the high temperature normally required in canning operations for that purpose. By carrying out sterilization in relativelyshort processing operations, without cooking the food in the accepted sense to a point at which it is ready to eat, without further cooking, it is possible to produce sterilized foods in Jars for example, which sterilization is sufllcient to prevent spoilage, leaving the foods to be cooked completely by the consumer at the time that they are to be consumed. In this way, it is possible to produce foods in a condition in which they can be readily distributed in only a partially cooked condition comparable in a way to the distribution of frozen foods without, however, the necessity of keeping them in a frozen condition.

In carrying out the processing by means of radio frequency heating, various types of apparatus may be employed. That illustrated in Figures 2 and 3'of the drawing is particularly useful in givinggood control especially where uniformity of heating is desired and particularly in connection with the processing of .foods although as indicated above, it may be employed in connection with various processes and methods including the carrying out of chemical reactions, etc., particularly where non-homogeneous solid and liquid mixtures are being subjected to a heating operation.

Referring to Figure 2 of the drawing, there is shown the container i of glass or other dielectric material which is positioned with respect to a plate electrode V2 anda ring electrode 3. The plate electrode 2 is placed adjacent the bottom of the container I while the ring electrode 3 is tion includes the step of starting the processing positioned at a distance from the plate electrode 2, and encircles the container I. For convenience, the vessel containing the material to be heated, is desirably cylindrical in shape, as illustrated in Figure 2; but it may take any other desirable shape such as rectangular, etc., with electrodes conforming accordingly. The invention is illustrated by the use of the cylindrical shaped vessel.

When the cylindrical container I is employed, the plate electrode 2 may take the form of a ilat circular plate of metal, while the ring metal electrode 3 may be carried on arm 4, supported on standard 5. The elements 4 and 5 are desirably of metal and serve as means for electrically connecting the ring electrode to the source of power. The arm I may be adjustably mounted on the standard 5 by means of thumb screw 6. Power may be supplied through the power lines 1, l.

To control the heating effect at the bottom of such vessel. as/for example, to overcome localization of heating or possible overheating, the vessel is desirably` spaced from the plate electrode. Air spacing may be employed, but under commercial conditions may oier mechanical difficulties. Where the container or vessel is to be placed adacent to such plate electrode, as for example, in being placed directly on such electrode, it is found desirable to utilize a material of low power loss factor between the bottom of the vessel or con- /tainer i and the plate electrode `2. Such low power loss factor material is indicated at 8 in Figure 2. The type of material used depends on the particular conditions of the operation being carried out. A block of sintered Pyrex glass of suitable thickness is fairly useful where the conditions are not such that there is rapid heating of such block ofmaterial. More desirably a ma- 7 terial oi' lower loss factor may be employed such as sintered or porous "Nonex glass or "Vycor" (96% silica) glass or quartz or volcanic rock or pumice stone or bers of these materials. A simple means that may be employed for heating stationary containers, as for example, in the heat treatment of smaller vessels or containers,

for example, in laboratories, consists in employing a piece of Mycalex" (mica and glass) sheet together with a folded piece of glass cloth to provide added spacing with a large percentage of air insulation. Such insulation between the .vessel or container i and the plate electrode 2 sufllciently controls the heating operations with respect to the vessel or container i and the electrode 2. Insulation of the ring electrode l is desirably by air spacing from' the upper part of the container.

The positioning of the ring electrode l with respect to the material undergoing heat treatment must also be taken into consideration since the skin effect" currents at a surface which has some degree of conductivity may cause local overheating. The positioning of the ring electrode, therefore, is important in producing the particular result desired. In order to obtain the best field distribution for uniform heating, the ring should be as high as possible, even above the surface of the batch of material undergoing treatment. Such positioning may be employed for example, in connection with the "steaming of!" process referred to above. In general, however, in those cases where too much surface heating takes place, the ring should be lowered until it is even with the surface or below the surface of the,

material undergoing heat treatment, depending on the conductivity of the batch, the desirability of heating the surface faster than the body, etc.

Thus by variation of the height adjustment of the ring electrode 3 (which may be done readily by hand or by a small electric motor geared suitably to an insulated shaft), means are provided for quickly, continuously, and for automatically adjusting and controlling the temperature of the upper part of the batch being heated.

The temperature control of the lower part of the batch has been partially described in discussing the means for spacing the vessel or container from the bottom electrode. This control has been found to be dependent to a large extent upon the dielectric loss factor of the container itself, because the heating rate of the batch in contact with the bottom depends upon whether the bottom heats faster or more slowly than the batch itself. The conductivity and dielectric loss factor of the batch of material, is also highly important in relation to the material of the container and so also is the material used for spacing the container and the distance of such spacing with respect to the plate electrode.

Thus by adjustment of the factors discussed above, almost any reasonable degree of control of temperature can be obtained for a given processing or treatment, especially for liquids and homogeneous materials but also for nonhomogeneous products including the sterilization of solid and irregular shaped pieces of foodstuffs, variations may readily be made. depending on the factors involved.

While the methods and apparatus described above have been based on the "symmetrical" output circuit of the oscillator or oscillator and amplifler wherein the two output electrodes remain at high potential, other methods and systems SAOBJBO may be employed. It is also good practice in radin frequency dielectric heating, to use only one high voltage electrode with the other electrode directly grounded. The two methods are practically interchangeable but one may be preferred for a given operation and should be considered with respect to particular equipment for such operation. The grounded electrode design is desirable in many cases for certain safety factors alone and is particularly true where the operations will be carried out without particular technical supervision.

I'he apparatus and methods may be utilised in commercial practice for continuous treatment of vessels or containers containing batches of materials to undergo heat treatment, as for example, glass jars of food. In such cases, the vessels or containers may be placed on a moving conveyor and carried through the heat treatment sone in which they are subjected to radio frequency heating. As illustrated in Figure 3, the containers il. i8 are placed on the continuous belt I9, operating over rolls 2li, 2t, and are fed forwardly into a heat treatment zone defined by the lower plate electrode 2l, contiguous to the lower portion of the belt I9. The latter is desirably made of insulating or dielectric material and carries the containers forward continuously over the electrode 2|. At the time that the containers reach a position over the electrode 2 l, they are encircled by the ring electrodes 2l, 2l, and power is applied to effect the heating during the passage of the containers through the heating zone over the electrode 2l. After the containers leave that zone, .the ring electrodes are removed and the containers passed then to storage or any other desirable treatment.

To illustrate the character of material and structure of the belt I9, it may be desirably of glass fabric with sintered glass or other low loss dielectric material cemented to it as for example, thin blocks 22, 22 containing in each of them a small depression or well 23 to carry the jar Il in a fixed position.

Various means may be utilized for lowering the ring type electrodes into position at the proper time, to maintain them in such position, and to remove them from the containers after the containers have passed through the heating zone. For example, a series of such ring electrodes may form a continuous chain in the shape of an endless belt arrangement, the electrodes dropping over the containers as they reach position A in Figure 3, and being removed as they reach position B, the length of the heat treatment zone being sufficient to carry out the particular processing employed. The rings and associated structure can be of very thin wall, light weight tubing carried in or on a moving glass cloth belt 2l which is long enough and loose enough on the rolls 2l to allow it to be guided into place over the iar position below. The rings may if desired, ride on rails of insulating material 21 as they are being lowered and these rails lmay be changed to metal 28 and connected to a high voltage electrode for the distance corresponding to the electrode below. In fact, the metal part of the "rail" may continue well past the bottom electrode if it is desirable to develop extra steam at the surface of the Jar just prior to sealing. This steaming off process is effected by bringing a single, high voltage electrode in proximity of the surface of the liquid and it apparently produces invisible corona in the surface vapor thus heating it rapidly. The ring will produce this effect if still at high potential as it is slowly lifted past the surface-of the liquid. The .iar may be sealed at this point, or have the liquid level adjusted. or simply removed from the belt. Such operations may be carried on continuously for sterilizationpurposes, for cooking operations, or for carrying out chemical reactions.

It may be pointed out that all of the processes set forth may be carried out in a pressure cham ber in order to quickly heat to any desired temperature above'the boiling point of water in order 'to get quicker and better sterilization, etc.'

This has been found'to be of especial value in processing non-acid vegetables.

As to the type of containers to be used, any desirable material may be employed that has the characteristics best for the purposes in hand. Reference has Abeen made above to glass bottles and it was found that Duraglas bottles were quite satisfactory in resisting heat shock. a consideration of importance from the standpoint of a commercial processing plant. However the utilization of glass bottles is not limiting, and any desirable containers may be used that adapt themselves for use under the particular conditions of processing involved.

Reference has been made above to Figure 1 of the drawings, showing a heating rate curve of sodium chloride. For comparative purposes there are also shown curves for sodium sulphate and for p-toluene sulfonic acid solutions in water. The pH and conductivity curves of the sulfonic acid are also show'n in that Figure 1 and of interest for reference. These curves show that the measured property of the solution is difierent in each case but there is an exhibition of properties comparable for certain purposes in connection with this invention.

In addition to the particular electrolyte and sugar solutions referred to herein, and in addition to the materials shown in Figure 1, it has been found that phosphoric acid and hydrochloric acid show behavior similar to that of the sulionic acid. Any material that dissociates may exhibit properties of interest in this connection so that conditions may thus be selected for their best heating concentration and fastest heating speed. While sugar solutions should not dissociate appreciably, indications are that the traces of inorganic impurities remaining in the sugar are the controlling factor in their heating. For example, a certain 50% sugar solution will heat at about the rate of .005% sodium chloride. If .01% sodium chloride be added, the time to reach boiling will be less than half, and with .02% sodium chloride added, the time to boil is onethird of the straight 50% solution. In other words, the behavior of a sugar solution toward such added salts is similar to that of pure distilled water although the curve may be somewhat dierent in shape and in its position on the graph. lIn this way it is possible to control the heating rate for-canning fruits in syrups as easily as in the case of foods in plain water vehicles.

An example has been given above of the heating of grapes suspended in salt solutions. Further details are of interest in showing how varying. conditions and concentrations affect the operation, and in indicating how control may be carried out to secure particular results. Thus,

Red Tokay grapes were processed in .005% sodium chloride solution. Under these conditions, the grape insides were forced out through the stem opening or burst the skin in to 30 seconds, while the surrounding medium was at 10 a temperature of 43 C. Temperature measured as quickly as possible at the interior of the pulp mass was to 95 C. The grapes apparently were selectively absorbing the power and heating very non-uniformly. Using an .01% salt solution, the action was similar but not as vigorous and the final water temperature was up to 57" C. Using .03% salt medium, the action was similar but final water temperature was 85 C. Power absorption fell off during this heating; indicatingthat this arrangement was about peak heating rate. A .1% salt medium was'still more uniform in heating both the solution and the grape interiors and there was much less trouble from bursting. At .5% salt mediumthe heating was much slower and most uniform. although a -choice somewhere between .1% and .5% may be best in a commercial process. Of course other considerations such as frequency, voltage gradient. etc., may call for slightly different concentration to achieve the desired result. But these operations show that reasonable control may be obtained. As stated above, boiling is less vigorous when power absorption is on the down trend during the heating period (corresponding to the higher salt concentrations).

A frequency of 20 megacycles was used in the above tests and in the following.

In processing vegetables, it has been found that a fairly pure water medium is suitable, varying in the heating rate range corresponding to about .003 to .02% sodium chloride (see Figure l) although in general it may be said that any concentration below about .1% sodium chloride may be used, but with slower results. (Normal city water supply is likely to fall in the .003 to .007% range.) The `low concentration was found desirable because mineral water salts are rapidly forced out of the pieces of food and the processing may end up by being too slow due to the salt concentration range increasing to the part of the curve where heating is slow. Low initial concentration was also found desirable to heat the pieces very rapidly internally to drive out air and form steam to quickly pull the salt solution into the food particles during cooling, so as to allow for sufficient added salt solution or water to leave the level covering the food after complete cooling. A second processing to boiling temperature was found desirable unless the initial processing and handling is done in a pressure chamber where the initial boiling temperature reaches C. or higher. The second processing is desirably done in a pressure chamber also in order to attain a higher temperature in the container.

The following examples will further illustrate the invention, but it will be apparent from what has been set forth above, that many variables in the process will be obvious depending for example on the heating rates and maximum temperatures desired.

A c. c. baby food jar is sterilized and packed with pieces of beans. 'I'hese are surrounded by .005% sodium chloride solution not quite covering all the beans. Heating is carried out in the ring and plate apparatus and brought to boiling in about 1 minute. The power is shut oil and boiling .005% salt vsolution is added to iill the jars to about 116 inch from the top edge of the bottle neck. The jar isvcapped and sealed. In 24 hours, the sealed jar is reprocessed in the same equipment until a slight bulge of the lid is noted, indicating pressure within the jar. This requires about 5 minutes but may be speeded up. The product is then ready for storage. The liquid 1l level will be about equal to the product level after the jar has cooled.

A second sample is processed similarly on first processing, but is processed 24 hours later in a pressure chamber under 20 lbs. air pressure until the lid again shows pressure inside. Since air has been practically 100% excluded, the pressure is largely steam and indicates a temperature close to that of steam at 20 lbs. pressure.

Vegetables processed as set forth above, have not been nearly Icooked in the usual sense and require about as much cooking by the housewife as do fresh vegetables, and samples have indicatedthatthetastewillbequitesimilartothe taste of fresh vegetables.

The methods of the present invention may be desirably applied in the rapid determination of impurities present in solutions. For example, in connection with the methods as described above where the heating of sugar solutions is set forth and the presence of even traces of inorganic impurities remainingin the sugar are a controlling factor in the heating, utilization of these factors may be made for analytical procedures. Thus it may be pointed out that power absorption rate as registered by a meter in the plate circuit of a suitable oscillator, or the heating rate as measured by a suitable temperature measuring device, can be used as a quick determination of impurities present in vsugar solutions or solutions of other non-electrolytes, and is capable of much greater accuracy when the slope of the curve is very steep, such as between and .02% salt concentrations.

It may be pointed out that the R. F. power applied to the product in accordance with the present invention, is essentially to condenser plates with the product as a dielectric. In fact conventional condenser plate arrangements can be used in many of the operations described herein, but the ring and plate arrangement described, has been found to be better for uniform heating where substantial uniformity is desired.

Having thus set forth my invention, I claim:

l. The method of heating food by radio frequency power which comprises immersing the food in an aqueous medium containing an added electrolyte in an amount not exceeding 1% to give said medium the dielectric properties desired for heating the particular food material when subjected to radio frequency power and subjecting the immersed food to radio frequency power to heat the food.

2. The method as set forth in claim l in which the electrolyte is salt in a concentration of from 3. The method as set forth in claim l, in which the liquid medium contains up to about 1% of salt.

4. The method of heating food by radio frequency power which comprises immersing the food material in an aqueous medium containing an added electrolyte in an amount not exceeding 1% to give substantially uniform heating with radio frequency power. and subjecting the immersed food to radio frequency power to heat the food.

5. The method as set forth in claim 4, in which the electrolyte is salt in a concentration of from about .02% to .5%.

6. The method of heating food by radio frequency power winch comprises immersing the food material in an aqueous medium containing an added electrolyte in an amount not exceeding 1% to give substantially maximum heating with rtnozed foodtoradiofrequency powertoheatthe 7. The method of heating food by radio frequency power which comprises immersing the food material in a liquid medium containing an added electrolyte in an amount not exceeding 1% where the power absorption decreases with increased temperature, and subjecting the imilimnrdsedfoodtoradiofrequencypowertoheattbs 0,'1'hemethodofheating foodbyradiofr quency power which comprises immersing the food material in a liquid medium containing an added electrolyte in an amount not exceeding 1% where the power absorption increases with increased temperature, and subjecting the immersedfm food toradiofrequencypowertoheattbe 9. The method of canning food material which comprises immersing the food material in a glass container in an aqueous medium containing an added electrolyte in an amount not exceeding 1% to give said medium the dielectric properties desired for heating the particular food material when subjected to radio frequency power, and subjecting the immersed food to radio frequency power to heat the food.

10. In a method of heating food by radio irequency power, the step which comprises immersing the food materialin an aqueous medium containing an added electrolyte in an amount not exceeding 1% to give said medium the dielectric properties desired for heating the food material more rapidly than the aqueous medium to expel air rapidly from within the food.

11. A method as set forth in claim 10, in which the container contents are cooled to lower the liquid level therein, further liquid is added to said container, and the container contents subjected to a steaming of! operation at the upper surface.

l2. A method as set forth in claim 10, in which the container contents are cooled to lower the liquid level therein, further liquid is added to said container, and the container contents subjected to a steaming of! operation at the upper surface. and sealing the container.

13. In a method of canning foods. the step of subjecting the food material in a container of dielectric material containing an aqueous medium in which the food material is immersed, to radio frequency power localized at the surface ofthe medium to generate steam at said surface.

14. A method as set forth in claim 10, in which the container contents are cooled to lower the liquid level therein, further liquid is added to said container. the container contents subjected to a steaming of! operation at the upper surface, and subjecting the container contents to a second heating by radio frequency power.

15. In a method of heating food by radio frequency power. the step which comprises immers ing the food material in an aqueous medium containing an added electrolyte in an amount not exceeding 1% to give said medium the iielectric properties desired for heating the food material more rapidly than the aqueous medium to expel air rapidly from within the food. cooling the container contents to lowerthe liquidlevel therein, adding further liquid medium to said container. sealing the container, and subjecting the container contents to a second heating by radio frequency power while said container is under superatmospheric pressure exradio frequency power, and subjecting the im- 7g ternai to said container.

` 16. In a method of heating food by radio i'requency power, the steps which comprise immersing the food material in an aqueous medium containing an added electrolyte in an amount not exceeding 1% to give said medium the dielectric properties desired for heating the food material more rapidly than the aqueous medium to expel air rapidly from within the food, cooling the container contents to lower the liquid level therein, adding further liquid medium to said container, steaming oi the upper surface of said liquid medium, subjecting the container contents to a second heating by radio frequency power, while said container is under super-atmospheric pressure external to said container.

17. In a method of treating food materials, the step of immersing the food material in a cold REFERENCES CITED The following references are of recordvin the file of this patent:

UNITED STATES PATENTS Name Date Schweizer Jan. 24, 1922 Hoermann June 14, 1932 Ball Aug. 31, 1937 Number 

